1. Field
This disclosure relates to optical systems and to stress-athermalized mounts for optical elements and optical elements suitable for mounting in stress-athermalized mounts.
2. Description of the Related Art
Optical systems are commonly constructed using transmissive elements such as lenses and filters mounted in primarily metal barrels or mounts. In this context, the term “transmissive” indicates the optical elements transmits a substantial portion, but not necessarily all, of the light within a wavelength band of interest. Transmissive materials used in optical elements include various glass, plastic, crystalline, polycrystalline, and glass-ceramic materials. Optical systems that are used outdoors or otherwise exposed to an extended range of temperatures must consider the effects of the different thermal expansion coefficients of the lens and barrel materials. A typical glass material used for optical elements has a thermal expansion coefficient of about 9×10−6 per C.°, which is to say that the physical dimensions of a lens fabricated from this glass will change by 9 parts per million for each degree Celsius temperature change. Other transmissive materials used for optical elements have thermal expansion coefficients ranging from under 0.6×10−6 per C.° for fused silica to more than 100×10−6 per C.° for certain plastics. Lens barrels and mounts are commonly fabricated from aluminum which has a thermal expansion coefficient of 24×10−6 per C.°. Lens barrels components may also be made from titanium alloys, beryllium alloys, copper alloys, magnesium alloys, various steels, or other materials, each of which has a unique thermal expansion coefficient.
For example, a glass optical element held tightly in a rigid aluminum barrel may become loose at a high temperature because of differential thermal expansion between the aluminum barrel and the glass optical element—the aluminum expands more than the glass as the system temperature increases. In contrast, a plastic optical element held in a rigid aluminum barrel may become stressed and deform or even fracture at a high system temperature, since plastic has a higher thermal expansion coefficient than aluminum. The opposite effects will occur at low system temperatures. The situation is more complex when a variety of transmissive materials are used in an optical system.
A conventional technique for alleviating the effects of differential thermal expansion is to retain optical components in a barrel or mount that incorporates a compliant member such as a spring, an elastomer O-ring or other gasket, or a flexible retainer or section of the barrel. The complaint member may than absorb the differential expansion and contraction of the various components. Compliant members may require a substantial space within the optical system. Further, the use of compliant members may still result in substantial stress in the optical components at temperature extremes and may also allow motion of the optical components under vibration and/or mechanical shock.
In this patent, an optical system in which the stress due to differential thermal expansion is alleviated or minimized for at least one optical element will be described as “stress athermalized”. An optical system in which the effect of temperature on one or more optical characteristics such as focal length, centering, field of view, distortion, or other characteristics is minimized will be described as “optically athermalized”.
Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having a reference designator with the same least significant digits.